What does this MR do and why?

This adds the capabilities to stream messages for GitLab Duo chat.

The streamed chunks get send over aiCompletionResponse but the order is not guaranteed on the client. Each chunk has a chunkId which has a guaranteed order, starting with 1.

The client_subscription_id is meant for the client to identify which mutation belongs to which subscription response. However, chat is a special feature where we want to sync responses between multiple chat instances of the same user.

This means we want to have two different subscriptions for the same request: 1. User-specific to sync between multiple clients 2. Request-specific for streaming

By not streaming the response when a client_subscription_id is not present, we also make sure that older clients still work when the stream_gitlab_duo is enabled. Otherwise, they would receive a streamed response on a subscription on user_id and resource_id.

NOTE: Related FE MR by @dmishunov !130347 (merged)

Screenshots or screen recordings

The following only works with the FE and graphql changes. There is an MR that combines both: !130415 (closed)

How to set up and validate locally

1. Have GitLab Duo and all AI features set up
2. Enable the FF stream_gitlab_duo
3. Checkout !130415 (closed) as it combines this MR with the FE changes and updates the GraphQL subscriptions

MR acceptance checklist

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